Bone metastasis occurs in 49% of patients with cancer of the breast. Although the diagnosis is currently made by x-rays and scintiscanning, there are no accurate indicators for the evaluation of the early response to therapy. Since bone matrix is the largest store of collagen, the invasion of bone by metastatic cancer is accompanied by an elevated excretion of hydroxyproline (HYPRO) in urine. HYPRO is excreted as dialyzable and non-dialyzable forms; the former represents breakdown of mature collagen, the latter is an index of synthesis of new collagen. It is therefore likely that serial determinations of dialyzable and non-dialyzable HYPRO can be an important tool in the evaluation of therapeutic response in patients with cancer of the breast with skeletal metastasis. We have demonstrated that the post-absorptive HYPRO/creatinine ratio (SPOT-HYPRO in morning urine) is an accurate index of bone involvement in cancer of the breast and multiple myeloma. In continuation of ongoing studies, we plan to determine: (1) the value of SPOT-HYPRO in the screening of patients at risk of developing bone metastasis from breast cancer; (2) if the evaluation of non-dialyzable HYPRO excretion, observed 24-48h after chemotherapy is an index of immediate response to therapy and if the test has prognostic value for long term treatment; (3) if there is correlation between the metastasis/normal bone ratio of densitometry in bone scans with the urinary excretion of HYPRO; (4) the validity of HYPRO measurements as an index of bone metastasis by determining the glucosylgalactosylhydroxylysine/galactosylhydroxylysine ratio in urine. This test differentiates the tissue of origin of the increased collagen turnover as bone or soft tissues.